Drillers and producers dislike the use of well-scanning tools that disrupt drilling and/or producing operations. They know that with the drill or producing string pulled from a wellbore and a scanning tool in place, many problems can arise.
For example, differential pressure at the contacting surfaces of the tool with the sidewall of the wellbore can generate a positive force as a function of time. As in-hole tool time increases, so does the likelihood of the tool becoming struck. Also, the drilling mud gets stiffer the longer the tool is within the wellbore, and accumulations on the top of the tool also build up. Such effects are complicating factors for clean removal of the tool even if the latter is continuously moving within the confines of the wellbore during data collection. So, the less time a tool is within the wellbore, the better the chances of its successful removal from the wellbore--on time.
In present NML tools, resident in-hole time has been dictated by requirements of the method itself as well as by system circuits for carrying out the method. For example, the NML data must be collected such that the effect of the polarization of the prior-in-time collecting cycle is essentially zero. Hence, either (i) sufficient time must be allowed between collection cycles, or (ii) the next-in-time polarizing period must be sufficiently long to establish maximum polarization of the entrained fluids before the NML data is collected.
Heretofore, commercial NML operations have provided sufficient conditions whereby conditions (i) and (ii) have been met. In the simplest NML mode of operation in which NML data is collected to establish the "free fluid index" of the formation fluids, the polarizing field is applied to the formation a sufficient time period that maximum polarization of the nuclei is established. That time period automatically guarantees that polarization of previous cycles will be at equilibrium before the proton precession signals are detected.
For cyclic NML operations, different steps are needed. A series of different polarizing time and collection time periods are used in association with a common given depth interval of formation (occurring in either T.sub.1 -continuous or T.sub.1 -stationary operations). Problems can occur when a cycle with a short polarizing period follows a cycle with a long polarizing period. As a result, polarization built up during the long polarizing period may spill over into the short period and may be manipulated by magnetic fields of the latter in such a way that the polarization buildup of the latter period does not start at zero. Hence, under these circumstances, heretofore a depolarizing time interval was inserted in the cycle of NML operations to allow the residual polarization to decay to equilibrium by relaxation. Such depolarizing time interval is of the order of two seconds. But since the polarizing periods of cyclic NML operations is each only a few tenths of a second and the signal observation intervals each is likewise only a tenth of a second or so, the need for such a long depolarizing period has imposed severe limitations on NML logging speed, say to about 300 feet/hour. It has only been tolerated because of the large time requirements of the uphole computer linked to the NML tool, viz., the time needed by that equipment to reduce the NML data to an acceptable display form. I.e., such reduction (being of the order of two seconds to reduce the observed data to an acceptable display form) has allowed time for the prior residual polarization to decay to equilibrium before the shorter polarization cycle was implemented.
However, now improvements in hardware and software within the associated uphole system at the earth's surface have been proposed by the field service companies. Goal: to reduce the time frame needed for the computer to reduce the NML data to acceptable form between collection cycles. Such advances encompass hardware, software and/or firmware improvements from individual as well as various combinational forms. However, I have found that the total time required for performing a set of three collection cycles of different polarizing periods (even though combined in a collection process that uses the above-mentioned proposed improvements), remains about the same as previously practiced. Reason: in cyclic NML logging, a 2-second depolarizing period must be used between selected collection cycles to insure that the polarization buildup always begins at zero. The speed of the logging sonde under these circumstances: about 300 feet/hour.
These limitations also apply regardless of how long the polarizing times are, or how the ratios of the polarizing periods relate one to the other. For example, in reference to the former in practicing T.sub.1 continuous logging even if the polarizing priods of a normalized set, were changed to 3200, 800, and 1600 milliseconds, the total time per sequence would still take 6 seconds. Even if the polarizing periods were changed to 3200, 800, and 1600 milliseconds, the total time per sequence would still take 6 seconds even though there is no need to insert depolarization periods between cycles.
Or in reference to the latter, instead of polarizing times of the set defining ratios of 4:1:2, different ratios, say 20:1:4 (viz., a set of polarizing times of 2000, 100, and 400 milliseconds with each followed a short signal-observing period) requires about 5 seconds per sequence, since a 2-second time for depolarization must be used after the 2000-millisecond polarizing period. Result: little improvement in logging speed.
Hence, there is now a need to artificially dispose of the effects of prior-in-time polarization within a period substantially shorter than the typical 2-second maximum mentioned above under normal NML cyclic operations and preferably within a period shorter than the present signal-observation time (viz., shorter than above one-tenth of a second). In that way, there would be provided a significant improvement in NML logging speed, e.g., say from 300 to about 600 feet/hour.
Hence, an object of this invention is to provide a method of reducing the effect of residual polarization in cyclic NML operations normalized to a common depth interval whereby such polarization can be reduced to approximately zero within a time period less than the present signal observation time, viz., less than approximately 100 milliseconds. Result: the repetition rate for a series of NML collection cycles normalized to the same depth interval is much improved and logging operations can be carried out at a surprisingly rapid rate.